Conventionally, a power conversion device such as an inverter which is able to convert DC power into AC power having an arbitrary frequency and voltage is widely used in a drive control of a rotary electric machine. The inverter turns on/off a semiconductor switching element on the basis of a voltage command value to output power having a desired frequency and voltage, and can control a rotation speed and a drive current of the rotary electric machine. In the control of the switching element of the inverter, PWM control is used in many cases to determine switching timing of the element using a pulse width modulation (PWM) signal.
As one of schemes of the PWM control, there is known an asynchronous PWM control in which a switching timing is determined by simply comparing a signal of the voltage command value with a signal of a carrier wave to generate a PWM pulse. In the scheme, the synchronization is generally not taken with respect to the frequency of the voltage command value since the frequency and the phase of the carrier wave are fixed.
In addition, as a technique other than the basic asynchronous PWM control described above, there are disclosed JP-2010-51129-A and JP-2006-204067-A.
In JP-2010-51129-A, there is an explanation “the synchronous PWM determiner 46 determines the number of times of synchronization in the synchronous PWM control, and the synchronous PWM phase controller 48 determines a frequency of a triangular carrier according to the number of times of synchronization, and determines timing of switching the number of times of synchronization. The PWM control unit 40 performs a synchronous PWM control in which the phase of the triangular carrier is synchronized with that of the voltage command.” (see ABSTRACT)
In addition, in JP-2006-204067-A, “the inverter control circuit determines a phase of a sinusoidal reference signal using the phase determination unit 28, outputs a target current by performing a feedback control only on a positive period of the converted AC current waveform, stores the waveform output during the positive period immediately before the negative period in a storage unit 25, inverts and outputs the waveform, and makes a control such that the amplitudes of the currents output in the positive and negative periods become equal.” (see ABSTRACT)